The present invention relates to CMOS integrated circuit technology. Specifically, a circuit for implementation in CMOS technology is described which controls the slew rate of pulses on a signal line.
Digital devices such as microprocessor implemented products have time-sensitive signal paths which carry digital signals. Signal transistors comprising a digital signal along a signal path have a unique time synchronization with devices which are connected to the signal paths. One of the ways of controlling the time of arrival of a digital signal on a signal path is to control the slew rate of the signal transitions. A circuit which acts in response to a rising or falling edge of a digital signal transition may have its response delayed by decreasing the slope, or slew rate, of the rising or falling edge of the digital signal.
Various devices have been employed in the prior art to control slew rate by switching in another device in a signal path if the slew rate is too slow, such as is shown in U.S. Pat. No. 5,015,880. Other techniques for controlling slew rate include a feedback method as exemplified in U.S. Pat. No. 5,619,147. The circuits described in these patents are utilized as driver circuits, which match the impedance and reduce reflections of signals which are propagated on a signal path extending off chip, and require additional CMOS transistors to implement the slew rate control.
Slew rate control is required when operating certain CMOS circuits under high stress conditions, such as during Burn-In. During high stress conditions such as Burn-In voltages and temperatures are increased and the digital signal propagation speed and slew rate tends to increase, sometimes interfering with operation of the circuit. Decreasing the slew rate under these circumstances may restore the operation of the circuit.
Control over the slew rate also permits a reduction in noise coupling between signal lines. The faster signal transitions tend to propagate to other signal lines introducing noise on the other signal lines. By reducing slew rates it is possible to decrease this particular source of noise.